Extensometers have been used to measure movement in naturally occurring rock structures, in coal mine roofs, and in foundations. Such changes are of basic scientific interest and of practical significance. Monitoring minute changes in naturally occurring fractures provides important information concerning the flow of ground water and potential transport of pollutants as well as the geological impact of either the extraction or injection of fluids into boreholes on fracture stability.
U.S. Pat. No. 5,929,341 issued to Bawden, et al on Jul. 27, 1999 describes and claims a device that indirectly measures stability of rock strata by measuring stress exerted on support cables positioned to support otherwise unstable material. The device finds particular application in the mining industry in which blocks of ore of a maximum dimension are removed potentially weakening remaining rock or with mining operations where the rock is inherently weak or fractured. The '341 invention addresses cable geometry and various systems to anchor the cable, such that elongation of the cable accurately reflects stress and the movement of rock.
U.S. Pat. No. 6,311,564 issued to Martin, et al. on Nov. 6, 2001 describes and claims an apparatus to provide support for a structure (i.e. rocks) and for measuring stress on the apparatus from the structure. The apparatus comprising an elongated center wire, several stress measuring devices, such as wire or other strain gauges positioned along the wire, a forming material encasing the center wire, several non-center wires extending longitudinally from the center wire and wound around the length of the center wire, stress measuring devices, and forming material, and a device to collect data. The apparatus is useful in measuring stress in the roof structure of a coal or similar, underground mine, or rock mass.
Capelle, et al. in U.S. Pat. No. 4,719,803 describe and claim improvements in a borehole extensometer. Compared with ten existing borehole extensometers, the '803 improvements eliminate the requirement of a surface reference head and the borehole extensometer is capable of providing in borehole displacement measurements.
U.S. Pat. No. 5,585,555 issued to McRea on Dec. 17, 1996 describes and claims a multiple position, recoverable borehole strainmeter. The device includes two or more anchors with releasable pistons that engaged the walls of the borehole to mount the strainmeter in the borehole. A relative displacement sensor senses changes in the relative displacement between adjacent anchors thereby measuring mass displacement axially along the borehole between the anchors. The pistons are independently, gas operated.
U.S. Pat. No. 5,629,480 issued to Herget on May 13, 1997 describes and claims an extensometer for use in a borehole. The device comprises a combination of linear motion transducers located with daisywheel anchors.
U.S. Pat. No. 4,607,435 issued to Boisen on Aug. 26, 1986 claims a temperature-compensated borehole extensometer. The device compensates for temperature effects on sensing rods by use of an element with materials of disparate linear coefficients of expansion.